Pneumatic reciprocatory actuator and method of operating it

ABSTRACT

In a pneumatic reciprocating actuator of the kind comprising a cylinder which defines a forward-motion chamber and a return-motion chamber, and a piston which is reciprocal in the cylinder and separates the forward-motion chamber and the return-motion chamber, a start stroke forwardly is initiated by admission of gaseous operating fluid from an inlet port in to the foreword-motion chamber with both the forward-motion chamber and the return-motion chamber initially at zero pressure. The operating fluid is also passed from the inlet port into the return-motion chamber during the start stroke until a back pressure of a predetermined magnitude has been produced in the return-motion chamber. Then the return-motion chamber is automatically closed by a pressure control valve unit, whereby the return-motion chamber will form a cushioning chamber for cushioning the movement of the piston in the forward direction, thereby preventing piston over speeding during the start stroke.

This invention relates to pneumatic actuators and, more particularly, toa reciprocatory actuator of the kind comprising a cylinder which definestwo cylinder chambers separated by a piston which is reciprocable in thecylinder.

When operation of such actuators, also referred to as linear motors, iscommenced, starting from an idle condition of the actuator with bothcylinder chambers at zero pressure, a first or start stroke of thepiston is initiated by admitting the operating fluid, i.e. air or othergaseous fluid, under pressure into one of the cylinder chambers. Thepiston then tends to move at excessive speed, because its movement isnot impeded by a back pressure in the other cylinder chamber. A backpressure will be present to prevent the piston from overspeeding ormoving at such excessive speed only after the first stroke of the pistonhas been completed.

To prevent such overspeeding of the piston it has been common practiceto initiate the operation of the actuator at a reduced pressure of theoperating fluid and raise the pressure to the normal working pressureonly after completion of one or more operating cycles.

Operating the actuator in this manner is disadvantageous in severalways. For example, the initial stroke or strokes are normallyineffective, and initial operation of the actuator at a reduced pressuremay be impracticable if a force corresponding to the normal operatingpressure is required from the actuator during the first stroke.Moreover, a pressure regulator is required for changing the initialreduced operating pressure to the normal operating pressure, and it maybe difficult to change the back pressure as desired to adapt it tochanges in the load to be driven by the actuator.

In accordance with a first aspect of the present invention there isprovided a method of operating a pneumatic reciprocatory actuator of thekind comprising a cylinder which defines a forward-motion chamber and areturn-motion chamber, and a piston which is reciprocable in thecylinder and separates the forward-motion chamber and the return-motionchamber, in which a start stroke of the piston in a direction expandingthe forward-motion chamber is initiated by admission of a gaseousoperating fluid from an inlet port into the forward-motion chamber withboth said chambers initially at substantially zero pressure,characterised by also admitting the operating fluid from the inlet portinto the return-motion chamber during the start stroke until a backpressure of a predetermined magnitude has been produced in thereturn-motion chamber, and then closing the return-motion chamber,whereby the return-motion chamber will form a cushioning chamber forcushioning the movement of the piston and thereby preventing pistonoverspeeding during the start stroke.

In accordance-with a second aspect there is provided a pneumaticactuator comprising a cylinder having an inlet port for operating fluidand defining a forward-motion chamber and a return-motion chamber, apiston which is reciprocable in the cylinder and separates theforward-motion chamber and the return-motion chamber, and a valve devicecontrolling flow of operating fluid into and out of the forward-motionchamber through a first fluid passage extending between the inlet portand the forward-motion chamber, characterised in-that the valve deviceincludes a pressure control valve unit which is disposed in a secondflow passage extending between the inlet port and the return-motionchamber and openable in response to the application to the inlet port ofa predetermined first pressure to pass operating fluid from the inletport to the return-motion chamber and closable in response topressurisation of the return-motion chamber by a predetermined secondpressure lower than the first pressure to block flow of operating fluidfrom the return-motion chamber.

Preferred embodiments of the method and the actuator according to theinvention include features set forth in the dependent claims.

The invention is applicable to actuators for various uses, such asactuators for use in reciprocating work machines or machines for openingand closing doors, actuators for use as pneumatic springs, pneumaticshock absorbers, etc.

In accordance with the invention, overspeeding of the piston during thefirst or start stroke is effectively avoided even if the full operatingpressure is immediately applied to the forward-motion chamber.

In accordance with the invention, the back pressure to be produced inthe return-motion chamber can also be adjusted rapidly and by simplemeans to suit different loads such that a lower back pressure isproduced for lighter loads and vice versa. If desired, the adjustmentcan be effected during operation of the actuator, e.g. automaticallyunder control by a load-sensing device connected to an adjustingmechanism.

Further features and advantages of the invention will become apparentfrom the following detailed description of an embodiment of the actuatoraccording to the invention.

FIG. 1 is a side view, largely in longitudinal section, of a linearpneumatic actuator embodying the principles of the invention;

FIG. 2 shows the rear end portion of the actuator of FIG. 1 drawn to alarger scale;

FIG. 3 is a cross-sectional view on line III—III of FIG. 1; and

FIGS. 4 and 5 are enlarged longitudinal sectional views of the rear endportion of the actuator in FIG. 1 and show an automatic pressure controlvalve arrangement, FIG. 4 showing a phase of the start stroke of theoperation of the actuator in which the operating fluid is being admittedinto both cylinder chambers and FIG. 5 showing a subsequent phase of thestart stroke in which the admission of the operating fluid into thereturn-motion chamber has been discontinued after the desired backpressure therein has been developed, while admission of the operatingfluid into the forward-motion chamber continues.

The linear pneumatic actuator or motor shown in the drawings comprises acylinder 1 and a piston 2 which is reciprocable in the cylinder andincludes a piston rod 3, the end of which protrudes from the cylinder. Asingle port 4 for connecting a fluid line leading from a source ofoperating fluid, typically air, is provided in a rear cylinder head 8 atthe rear end of the cylinder. From this port 4, operating fluid underpressure can be passed into into a first chamber 5, hereafter referredto as the forward-motion chamber, in the cylinder 1. A port 6 inconstant open communication with a second cylinder chamber 7, hereafterreferred to as the return-motion chamber, is also provided in the rearhead 8. As shown in more detail in FIGS. 4 and 5, the cylinder head 8houses a pressure control valve unit, which will be described below.

As shown in FIG. 3 the wall 9 of the cylinder 1 is star-shaped witheight star points 10, all of which are formed with elongate passages inthe shape of axial holes 11 extending throughout the length of thecylinder. Four of the holes are used for the reception of tie rods 12,and one or more 11 a of the remaining holes may be used for conveyingcompressed air, especially in the type of cylinder in which there isonly a single connection for a source of compressed air. A frontcylinder head 13 at the right-hand end of the cylinder 1 has a throughbore for the piston rod 3. Adjacent that cylinder head 13 an aperture 14interconnects the return-motion chamber 7 of the cylinder 1 with theuppermost one 11 a of the axial holes 11. A similar aperture 15 adjacentthe left-hand or rear cylinder head 8 interconnects the forward-motionchamber 5 with the lowermost one 11 b of the axial holes 11. The axialhole 11 b, which is plugged adjacent the front cylinder head 13, formsan extension of the forward-motion chamber 5 to increase the totalvolume thereof.

Opposite sides of the piston 2 are provided with an extension forming adamping piston 16, 17. Each damping piston cooperates with a respectivedamping cylinder 18, 19 at opposite ends of the cylinder 1. The dampingpiston/cylinder devices 16, 18 and 17, 19 serve to cushion the strokesof the piston 2 adjacent the cylinder ends.

The above-mentioned pressure control valve unit comprises twocooperating valves 20 and 21 which control the flow of operating fluidunder pressure from the port 4 into the return-motion chamber 7. Thevalve 20 serves to isolate the return-motion chamber 7 from the fullpressure of the operating fluid once the predetermined back pressure hasbeen reached in that chamber, and the valve 21 serves to admit operatingfluid into the return-motion chamber 7 until that back pressure has beenreached.

A valve compartment in the valve 20 accommodates a valve member 22 whichis urged in the direction of the valve 21 by a compression spring 23 andthereby biassed towards a closed position in engagement with anassociated valve seat in the form of an annular seal. A spigot 24 on oneside of the valve member 22 extends into a valve chamber of the valve 21in which a valve member 25 is accommodated and urged in the direction ofthe valve 20 by a compression spring 26 and thereby biassed towards anassociated valve seat formed by a wall of the valve chamber. Thebiassing force of the spring 26 can be set by turning an adjusting screw27 belonging to the valve 21.

A flow passage 28 for operating fluid extends from the port 4 and opensinto the valve compartment in the valve 20 through a restricted orifice29. Extending from the port 4 is also a flow passage V1 which opens intoa two-position directional control valve V. In a first position thereof,the valve V passes operating fluid from the flow passage V1 into afurther flow passage V2 and through a port 30 opening into the lowermostaxial hole 11 b, from which the operating fluid passes into theforward-motion chamber 5 through the aperture 15. In a second positionthereof, the valve V interconnects the flow passage V2 with a flowpassage V3 to discharge operating fluid from the forward-motion chamber5 and the hole 11 b.

Naturally, the just-described arrangement comprising the three-way,two-position valve V and the associated flow passages V1, V2 and V3 onlyis an illustrative example of means and ways of pressurising theforward-motion chamber 5.

The diameter of the disc-like valve member 22 of the valve 20 is smallerthan the diameter of the valve compartment accommodating it. In the openposition of the valve member 22 shown in FIG. 4, air entering the valvecompartment can therefore flow past the valve member 22 through theassociated annular valve seat and then through an annular gap around thespigot 24. The spigot 24 is of such a length that when the valve member25 of the valve 21 is in the closed position shown in FIG. 4, it bearsagainst the end of the spigot to keep the valve member 22 in the openposition.

A flow passage 31 connects the valve chamber of the valve 21 with theaxial cylinder passage 11 a and thereby with the return-motion chamber 7through the aperture 14 such that the return-motion chamber 7 is alwaysin open communication with the valve chamber.

The operation of the actuator shown in the drawings is as follows.

FIG. 4 shows a situation in which a start stroke, i.e. a forward strokeof a first cycle of repetitive operation of the actuator, has beeninitiated by opening the supply of pressurised operating fluid to theport 4 with the valve V in its first position and both cylinder chambers5 and 7 initially at zero or very low pressure.

Operating fluid flows from the port 4 through the passages V1 and V2into the axial passage 11 b and through the aperture 15 into theforward-motion chamber 5 as indicated by arrows. At the same time,operating fluid flows through the passage 28 into the second valve 20and past the second valve member 22 to the front face of the valvemember 25 of the first valve 21. Initially, the pressure of theoperating fluid is sufficient to overcome the bias of the spring 26 anddisplace the valve member 25 slightly from the associated valve seat sothat operating fluid can flow past the valve member 25 and through thepassage 31 and the axial cylinder passage 11 a as indicated by an arrowand into the return-motion chamber 7.

Accordingly, both the forward-motion chamber 5 and the return-motionchamber 7 will be pressurised, although the back pressure thus producedin the return-motion chamber 7 by the admission of operating fluidthrough the valves 20 and 21 will be limited to a value substantiallylower than the maximum pressure produced in the forward-motion chamberin normal operation of the actuator, such as, for example, 10-20 percentof that pressure. For example, with a maximum operating fluid pressurein the forward-motion chamber of 7-10 bar, the admission ofoperating-fluid past the valves 20 and 21 may be discontinued when theback pressure in the return-motion chamber 7 reaches a desired value inthe range of 1-3 bar.

The maximum back pressure that can be produced in the return-motionchamber 7 is determined by the valve 21, namely by the setting of theadjusting screw 27 and thus the biassing force applied to the valvemember 25 by the spring 26. When the back pressure reaches the desiredset magnitude, the pressure acting on the front face of the second valvemember 25 will cause the valve member 25 to move away from itsassociated seat sufficiently to allow the biassing spring 23 of thevalve 20 to move the second valve member 22 into sealing engagement withits associated valve seat. The pressure of the operating fluid in thevalve compartment of the valve 20 acting on the valve member 22 willthen keep that valve member firmly engaged with its seat so that theoperating fluid trapped downstream of the valve 20 cannot escape.

The back pressure produced in the return-motion chamber 7 by the trappedvolume of operating fluid will act in opposition to the pressure in theforward-motion chamber 5 which displaces the piston 2 forwardly througha power stroke to extend the piston rod 3. The back pressure thus willprevent the piston from overspeeding under the action of the pressure inthe forward-motion chamber 5.

When the valve V is then placed in its second position to dischargeoperating fluid from the forward-motion chamber 5, the trapped volume ofoperating fluid in the return-motion chamber 7 will expand and displacethe piston 2 through a backward or return stroke to retract the pistonrod 3. When the valve V is then returned to the first position toinitiate a new forward power stroke, the back pressure already exists sothat it need not be produced again. Thus, once the back pressure in thereturn-motion chamber has been built up to the desired set level, thevalve 20 normally remains closed to isolate that chamber from the port4.

If for one reason or other the back pressure in the return-motionchamber 7 should fall below the desired set pressure, or if the settingof the back pressure is raised by means of the adjusting screw 27, thebiassing spring 26 will displace the valve member 26 towards theassociated seat to cause the valve member 22 of the second valve 20 toopen again and admit additional operating fluid through the valves 20and 21 until the back pressure is again at the desired set value and thefirst valve 21 recloses.

It may be desirable to adjust the maximum back pressure if the loadbeing moved by the actuator is changed so that the back pressure will beadapted to the load. Such adjustment can be effected rapidly and in asimple manner, even during operation of the actuator: to reduce themaximum back pressure the adjusting screw 27 is turned outwardly toreduce the biassing force applied by the biassing spring 26, and viceversa. By suitable adjustment of the back pressure, optimum operation ofthe actuator can be achieved, and in use of the actuator for closing andopening doors, as air spring suspensions for vehicles and in severalother applications, the operating and back pressures can be readilyadjusted for maximum safety and comfort.

The valve V or any other valve used for controlling the admission ofoperating fluid into the forward-motion chamber 5 suitably is arrangedsuch that the forward piston motion is reversed when the piston 2 hastraversed a predetermined distance in the cylinder.

What is claimed is:
 1. A method of operating a pneumatic reciprocatoryactuator (1-3) of the kind comprising a cylinder (1) which defines aforward-motion chamber (5) and a return-motion chamber (7), and a piston(2) which is reciprocable in the cylinder (1) and separates theforward-motion chamber (5) and the return-motion chamber (7), in which astart stroke in a direction to expand the forward-motion chamber isinitiated by admission of a gaseous operating fluid from an inlet port(4) into the forward-motion chamber with both the forward-motion chamber(5) and the return-motion chamber (7) initially at substantially zeropressure, characterised by also admitting the operating fluid from theinlet port (4) into the return-motion chamber (7) during the startstroke until a back pressure of a predetermined magnitude has beenproduced in the return-motion chamber (7), and then closing thereturn-motion chamber (7), whereby the return-motion chamber (7) willform a cushioning chamber for cushioning the movement of the piston (2),thereby preventing piston overspeeding during the start stroke, theadmission of the operating fluid from the inlet port (4) into thereturn-motion chamber (7) being effected through a pressure controlvalve device (20, 21) adapted to close a flow passage (28, 29, 31)between the inlet port (4) and the return-motion chamber (7) in responseto the pressure in the return-motion chamber (7) reaching thepredetermined magnitude.
 2. A method according to claim 1, characterisedby venting the forward-motion chamber (5) upon completion of the startstroke while maintaining the closed condition of the return-motionchamber (7), whereby the return-motion chamber (7) is allowed to expand.3. A pneumatic actuator comprising a cylinder (1) having an inlet port(4) for operating fluid and defining a forward-motion chamber (5) and areturn-motion chamber (7), a piston (2) which is reciprocable in thecylinder and separates the forward-motion chamber (5) and thereturn-motion chamber (7), and a valve device (V, 20, 21) controllingflow of operating fluid into and out of the forward-motion chamber (5)through a first fluid passage (V1,V2) extending between the inlet port(4) and the forward-motion chamber, characterised in that the valvedevice (V, 20, 21) includes a pressure control valve unit (20, 21) whichis disposed in a second flow passage (28,29,31) extending between theinlet port (4) and the return-motion chamber (7) and openable inresponse to the application to the inlet port (4) of a predeterminedfirst pressure to pass operating fluid from the inlet port (4) to thereturn-motion chamber and closable in response to pressurisation of thereturn-motion chamber (7) by a predetermined second pressure lower thanthe first pressure to block flow of operating fluid from thereturn-motion chamber (7), and in that the pressure control valve unit(21, 20) includes a first valve (21) having a first valve member (25), afirst valve seat associated therewith, and a spring (26) biassing thefirst valve member (25) to a closed position in sealing engagement withthe second valve seat, said first valve member (25) being displaceableby said predetermined first pressure to an open position against thespring bias, a second valve (20) including a second valve member (22), asecond valve seat associated therewith, and a spring (23) biassing thesecond valve member (22) to a closed position in sealing engagement withthe second valve seat, and a valve operating member (24) for keeping thesecond valve member (22) in an open position by means of the first valvemember (26) when the first valve member is in the closed position andallowing the second valve member (22) to move to the closed position inresponse to the first valve member (25) becoming displaced to the openposition.
 4. A pneumatic actuator according to claim 3, in which thevalve operating member (24) comprises a push rod positioned between thefirst and second valve members (25, 22) and in which the first andsecond springs (26, 23) bias the valve members in opposite directions.5. A pneumatic actuator according claim 3, in which the pressure controlvalve unit (21, 20) includes a valve adjusting device (27) for varyingthe first pressure.
 6. A pneumatic actuator according to claim 5 whendependent on claim 4, in which the valve adjusting device comprises anadjusting screw (27) for varying the bias of the first spring (26).
 7. Apneumatic actuator according to claim 3 in which the piston is providedwith a piston rod (3) protruding from one end of the cylinder (1) andthe opposite end of the cylinder is provided with a cylinder head (8),and in which the pressure control valve (21, 20) unit is incorporated inthe cylinder head (8).
 8. A pneumatic actuator according to claim 8having a single inlet port (4) for operating fluid, in which the inletport (4) is provided in the cylinder head (8) and the wall of thecylinder (1) is provided with an axially extending passage (11 a)communicating with the second flow passage (28, 29, 31) and thereturn-motion chamber (7).
 9. A pneumatic actuator according to claim 3in which the wall of the cylinder (1) is provided with at least oneaxially extending passage (11 b) communicating with the inlet port (4)and the forward-motion chamber (5).